The present invention relates to a process for the recovery and reuse of sodium and sulfur chemicals in connection with sulfate cellulose production, whereby the chlorine chemicals are simultaneously removed from the liquor system in the form of sodium chloride.
There are advanced processes for the recovery and reuse of digestion chemicals. So far, bleaching chemicals have not been recovered to a noteworthy extent. This is unfortunate, considering environmental pollution caused by the chemicals used for bleaching and by dissolved organic materials when they are discharged into the sewage system.
It is possible to use the waste water flow from the bleaching apparatus either entirely or partially for washing the digestion chemicals from cellulose. Besides, the waste flow from the bleaching apparatus can be used for dissolving the melt coming from the soda-ash roaster. The waste water coming from the bleaching apparatus can, furthermore, be used instead of condensation water and clean water at several other stages of the recovery system for digestion chemicals, not mentioned here.
The waste water flow coming from the bleaching system can be purified and then released into the sewage pipe. The purification can be performed by, for example, an ion exchange or calcium hydroxide treatment. In this case, however, there is the problem of what could be done with the chemicals and organic compounds removed from the waste water. In both of the methods for purifying waste water mentioned above, the purpose is to feed these chemicals and organic compounds into the recovery system for digestion chemicals. The waste water flows of the bleaching apparatus can be purified in other ways as well, and the removed substance can be led into the digestion chemical system.
In the recovery and purification systems for the bleaching system waste water described above, organic and inorganic substances are fed into the digestion chemical system. Various substances can be fed into the digestion chemical system even in other ways. The combustible substances are burned either in a soda-ash roaster or in a caustic-sludge furnace and then removed from the system. The incombustible substances, such as sodium chloride, remain mainly in the liquor system, unless they are removed or unless they are eliminated from the system in the form of losses incurred during the process.
When cellulose is bleached by means of oxygen gas, sodium is fed into the digestion chemical system along with the bleaching residues at either the recovery or the purification stage.
In chlorine bleaching, chlorine and possibly sodium are fed into the system at either the recovery or the purification stage. The present invention relates to a process for removing chlorine, sodium and sulfur from the system, whereby sulfur-free and chlorine-free sodium hydroxide is simultaneously produced for bleaching or the other unit processes. The invention makes it possible to control the sulfide content of the digestion solution in a simple manner.
There are also other processes for removing chlorides from the liquor system. Four processes are described below, but only in Process 3 has the sodium and sulfur balance control been taken into consideration.
1. Leaching of the chloride-containing dust from the soda-ash roaster. This process produces, however, a white liquor with a chloride content many times that produced by the process according to the invention.
2. Removal of sodium chloride from the white liquor by means of evaporation crystallization. The chloride content obtained for the white liquor by the process is about 3 times that obtained by the process now proposed. The evaporation requirement is, however, more than twofold. The greatest disadvantage of the process is the great carbonate quantity which is precipitated together with the chloride crystals and which must be recovered by leaching.
3. Separation of the carbonate and chloride from the green liquor by evaporation crystallization. After separation, the crystals are dissolved and the solution is causticized, whereafter the chlorides are removed by evaporation crystallization. The process has the same disadvantage as Process 2. Besides, the evaporation requirement is even greater than in Process 2.
4. Separation of the carbonate and chloride from the melt by leaching. This process is hardly practicable for the reason that the leaching is very difficult to perform.